Mail pieces (in particular letters) to be sent in large quantities are for the most part automatically sealed and franked by the sender. Among other things, sealing devices and franking machines are used for this, wherein both components are often connected with each other and sometimes form structural units. As an example, reference is made to the citation DE 20 2004 011 390 U1.
In connection with the sealing of mail pieces whose sealing flaps are provided with a gumming, it is necessary to wet the gumming before the sealing. For this a sealing liquid is (automatically) applied to the gumming. A plurality of technological problems are encountered in these contexts [sic]. On the one hand, the sealing liquid must sufficiently wet the gumming. Given an insufficient wetting, the sealing liquid will roll off and ultimately there will be no usable bonding is obtained. On the other hand, the amount of sealing liquid applied to the gumming needs to be measured such that on the one hand the gumming adheres, however on the other hand an undesired washing-out or smudging of excessive sealing liquid on the mail piece envelope after sealing does not occur. The latter is important in particular for the prevention of washing-out or smudging of water-sensitive inscriptions or imprints (such as franking stamp imprints) on the mail piece or the mail pieces situated above or below. This has the consequence that the dosing of the sealing liquid is a critical property in the sealing process.
In addition to this, depending on the system sealing devices start with too much water when a sponge of the sealing device is filled with the sealing liquid upon activation of the sealing device. Only after a start-up number of sealed mail pieces does an equilibrium with regard to the amount of sealing liquid contained in the sponge arise in the further (quasi-)continuous operation. As a result, at the beginning of a series of sealings the dose of the sealing liquid will normally be too high since the equilibrium dosage would in turn be too low given adjustment to an initially appropriate dosing. Particularly with the first mail pieces there therefore exists the significant danger of the excess of sealing liquid, with the consequence of the danger of washing-out water-sensitive imprints such as franking stamp imprints.
Furthermore, it is problematic that in practice mail pieces (in particular letters) whose mail piece envelopes or, respectively, letter envelopes are made from paper materials with varying material properties are fed to a sealing device. Paper materials of a low density can thus be significantly more absorbent than paper materials with high density. For a reliable and reproducible sealing of mail pieces with varying paper materials of the mail piece envelope as well, it would therefore be necessary to dose the amount of sealing liquid according to the requirement of the pertaining paper material. Given a material with high absorbency, a comparatively large quantity of sealing liquid would be necessary, since otherwise the sealing liquid would be absorbed quickly and the bonding would not turn out to be sufficient. Given a material of low absorbency, a comparatively small amount of sealing liquid would be required since otherwise the sealing liquid would not be sufficiently absorbed and a washing-out and running of excessive sealing liquid would threaten, with the disadvantageous effects described in the preceding.
Various sealing liquids are known from experience. These are aqueous solutions that on the one hand can contain biocide and fungicide in order to prevent the development of germ colonies and unpleasant odors. On the other hand, a wetting agent can be added. In practice, rinsing agent as a wetting agent is also added by users, for example.
The sealing liquids known insofar satisfy all the requirements since a dosing suitable for all mail pieces, and in fact from the beginning, cannot be set in practice and, depending on the number and kind of the mail pieces, a plurality of insufficient sealings and/or washing-outs is to be assumed.
Furthermore, in a different technical field (the production and sealing of cartons) gummed package tapes or, respectively, mounting tapes are used by means of which cartons are sealed or, respectively, fixed in the shape of a container. The application of these package tapes or, respectively, mounting tapes also often ensues in an automated manner, such that in principle the problems described in the preceding arise in this field.